C172 PLT GIVES INCORRECT CLRNC READBACK WHICH IS NOT CAUGHT BY CTLR RESULTING IN A CONFLICT WITH A DEPARTING MD80.

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CESSNA: SJC CLRNC; CESSNA XX VFR RHV WITH ATIS INFO. SJC CLRNC DELIVERY: VFR RHV R DOWNWIND; MAINTAIN VFR. CESSNA: R DOWNWIND; MAINTAIN VFR. SJC CLRNC DELIVERY: CESSNA READBACK CORRECT. TWR: L DOWNWIND CLRED FOR TKOF; CAUTION WAKE TURB MD80 DEPARTING. CESSNA: ROGER; CLRED FOR TKOF L DOWNWIND. I TOOK OFF FROM RWY 29 (THINKING ABOUT THE TURB I MIGHT ENCOUNTER WITH THE PREVIOUS DEP OF AN MD80). AT AN ALT OF APPROX 500 FT MSL; I STARTED MY TURN TO THE L. ABOUT 10-15 DEGS INTO THE TURN; I RECEIVED A CHANGE TO MY DEP CLRNC OF A R DOWNWIND DEP. I ACKNOWLEDGED AND TURNED R; XING THE FIELD. AFTER I WAS APPROX 1/2 MI E OF THE RWYS; TWR CAME BACK AND SAID I MADE A MISTAKE WITH THE R DOWNWIND DEP. I THINK I CAME BACK WITH AN APOLOGY AND THEY LEFT IT AT THAT; THEN WAS HANDED OFF TO RHV TWR FOR A NORMAL APCH AND LNDG. AT RHV I WAS REQUESTED BY THE TWR TO RPT WHEN I WAS READY TO COPY A SPECIAL REQUEST. WHEN I FINALLY GOT THE ACFT STOPPED AND SHUT DOWN FOR A MECHANICAL INSPECTION I COMPLETELY FORGOT TO CALL THE TWR. AFTER 45 MINS TO 1 HR; A COUNTY TRUCK CAME UP AND ASKED FOR THE PLT OF MY ACFT. I TOLD HIM THAT I WAS THE PLT. HE SAID TO CALL THE SJC TWR AND GAVE ME THE PHONE NUMBER. I TRIED TO CALL FOR ABOUT 20-30 MINS -- THE LINE WAS BUSY. I THEN DECIDED TO FLY BACK TO SJC TO TRY TO CALL THEM AGAIN. THAT'S WHEN I FOUND OUT WHAT HAD HAPPENED. I HAD A MEETING WITH THE SJC TWR TRAINING COORDINATOR AND WE REVIEWED THE TAPES. MOST OF WHAT I HAD REMEMBERED AND DESCRIBED WAS APPROX CORRECT. AT ABOUT 500 FT MSL WHEN I STARTED MY TURN L AND THOUGHT I RECEIVED ANOTHER CHANGE (R TURN); THE TWR ACTUALLY SAID; '...CONTINUE L 270 DEGS; CROSS MIDFIELD.' MY ACKNOWLEDGEMENT WAS 'ROGER STARTING R TURN.' WE DID DISCUSS A COUPLE OF WAYS TO POSSIBLY ELIMINATE THIS TYPE OF ERROR FROM HAPPENING AGAIN. THE FIRST IS TO WHENEVER YOUR DEST IS ON THE OTHER SIDE OF THE FIELD THAN FROM RWY 29; A L 270 DEGS; CROSS MIDFIELD TYPE OF CLRNC WOULD BE IN ORDER; FROM RWY 30R 29 A R 270 DEG; CROSS MIDFIELD TYPE OF CLRNC WOULD BE GIVEN. THE OTHER OPTION WOULD BE THAT CLRNC DELIVERY WOULD NOT GIVE ANY DEP INSTRUCTIONS LIKE 'R DOWNWIND OR L DOWNWIND DEP' SO THE PLT WOULD NOT POSSIBLY HAVE A MINDSET WHEN GND OR TWR AMENDS THE CLRNC.